Tungsten powder bonded filament connection for incandescent lamps and method of manufacture



Dec. 16. 1969 K. S. G. PERTWEE TUNGSTEN.POWDER BONDED FILAMENTCONNECTION-TOR NCAN ESCENT LAMPS AND METHOD OF MANUFACTURE Filed Feb.13, 1967 Irwvesrvtor: KTTOBC1IH 5.9.1 6 twee His m- -tne United StatesPatent O U.S. Cl. 313-331 4 Claims ABSTRACT OF THE DISCLOSURE Theconnection between a tungsten filament and a refractory metal lead wireis firmly bonded together by a mass of material consisting solely offine tungsten powder particles sintered by a surface tungsten bond toeach other and to the filament and the lead wire, and prepared byforming a suspension of very fine tungsten powder in a tungsten resinatesolution containing a suitable binder, applying a quantity of thesuspension to the connection and heating to a temperature of 1600l800 C.in a hydrogen atmosphere to decompose and drive oif all the organicmaterial in the suspension.

BACKGROUND OF THE INVENTION Field of the invention This inventionrelates to electric incandescent lamps comprising a sealed bulb orenvelope containing a tungsten filament connected to current supplyconductors through which current is conveyed from terminals at theexterior of the bulb. More particularly, the invention relates to thejoint or connection between the ends of the filament and the currentsupply conductors.

Description of the prior art The filaments of incandescent lamps aregenerally connected to the current supply or lead-in conductors by amechanical joint as opposed to a welded joint which tends to causeembrittlement and weakening of the tungsten filament adjacent to thejoint, with consequent breakage. In certain types of lamps asatisfactory joint is made simply by securely clamping a hook in the endof the current supply conductor about the end of the filament. However,in other lamp types, particularly higher wattage lamps, such a joint isnot feasible because the conductor is necessarily of larger size and isusually also made from more refractory and harder metal. The art hastherefore resorted to joints of rather complicated structure in order tosecure a satisfactory mechanical and electrical connection. However, incertain cases the frictional connection is unsatisfactory since itpermits the filament to move and also permits a certain amount of arcingand local overheating at the joint. Such arcing produces a noise which,in the case of projection lamps used in movie or television studios, ispicked up by nearby microphones.

More recently, as disclosed and claimed in my US. Patent 2,961,568, afirm conductive cohesion between the filament and conductor has beenachieved by a bond formed by application of a tungsten-platinum slurryto the connection, followed by heating at a high temperature to melt theplatinum which alloys with the tungsten 3,484,644 Patented Dec. 16, 1969and becomes consolidated to firmly join together the filament and theconductor. While such a construction gives good results, the presence ofthe platinum makes it rather expensive. Also, in order to achieve properflow and consolidation of the mixture, a quite high temperature (1850C.) is required which can cause premature failure of the furnaces inwhich the product is heated. In some cases, accidental application ofthe slurry to portions of the filament proper which operate at very hightemperatures, causes an attack or alloying of the platinum with thefilament with resulting embrittlement.

SUMMARY OF THE INVENTION It is accordingly an object of the presentinvention to provide a firmly bonded connection of the filament andlead-in conductor which is less expensive than prior art bonds. It is afurther object to provide a bond and method of manufacture thereof whichmay be formed at somewhat lower temperatures. It is a still furtherobject to provide a bond wherein the bonding material consists solely oftungsten.

Therefore, in accordance with the invention, the filament and conductorare firmly joined or bonded by a mass of material consisting solely offine tungsten powder particles sintered by a surface tungsten bond toeach other and to the filament and the conductor. In further accord withthe invention the bond is formed by applying to the connection a pasteor suspension of the fine tungsten powder particles in a tungstenresinate solution, and heating the connection in a non-oxidizingenvironment to a temperature sufficient to decompose the resinate andeffect a sintering of the tungsten particles to each other and to thefilament and the conductor.

The absence from the bond of any metal other than tungsten not onlyresults in a less costly construction and process, and avoidscontamination of the tungsten filament, but it also makes the bondsuitable for use in lamps of the halogen regenerative cycle type whereit is desirable to avoid the presence of metals other than tungstenwhich could react with the halogen and interfere with the regenerativecycle.

Further features and advantages of the invention will appear from thefollowing description of species thereof and from the drawing.

BRIEF DESCRIPTION OF THE DRAWING In the drawing, FIG. 1 is an elevationof a projection type incandescent lamp in which the invention may beembodied;

FIG. 2 is a fragmentary perspective view of the filament connection inthe FIG. 1 lamp; and

FIG. 3 is a side view, in section and on an enlarged scale, of one endof a form of halogen cycle type lamp having a filament connectionembodying the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The lamp shown in FIG. 1 is aconventional type high wattage projection lamp such as that shown in myaforesaid Patent 2,961,568, and comprises a sealed glass bulb orenvelope 1 containing a planar type filament 2 composed of a pluralityof coiled tungsten wire segments arranged in side-by-side relation. Theserially connected segments of the filament 2 are suitably supported,and the filament terminates, at each end, in a coiled leg portion 3which is fitted over an auxiliary current supply conductor or spud wire4 of refractory metal such as molybdenum, although wires of tungsten,tantalum or columbium may also be used. In the particular caseillustrated herein the spuds 4 have flattened end portions 5 (FIG. 2)fitted snugly in the coiled filament legs 3. The spuds 5 are connectedto respective current supply or lead-in conductors 6 which have portionsthereof sealed in and extending through a reentrant glass stem 7 whichis sealed to the lower end of the bulb 1.

It is highly desirable, and in many applications necessary, thatconnection of the filament legs 3 to the spuds or conductors 4 be firmlyfixed to avoid relative movement therebetween with consequent sparkingor arcing and local overheating. In accordance with the presentinvention, the filament leg 3 and end portion 5 of conductor 4 arefirmly bonded together by a mass of material indicated at 8 andconsisting solely of finely divided tungsten powder particles firmlysintered by a. surface tungsten bond to each other and to the filamentleg 3 and to the end 5 of conductor 4, all as more fully describedhereinafter.

FIG. 3 is illustrative of a halogen regenerative cycle type lamp such asthe iodine cycle type described and claimed in Patent 2,883,571 toFridrich and Wiley. The lamp illustrated is of the double-ended tubulartype comprising a tubular envelope 11 of high melting point vitreousmaterial such as quartz or fused silica containing a tungsten filament12 which may be of helically coiledcoil form having at each end thereofa helically singlecoiled leg portion 13 the end of which is fittedsnugly over the end of an inner lead-in conductor or spud wire 14 oftungsten. The spud wire 14 is connected to a molybdenum foil 15, towhich is also connected an outer lead wire 16 and which is hermeticallysealed in a pinch seal 17 at the end of the envelope. The envelope 11contains a filling of inert gas and a quantity of halogen such as iodineor bromine, preferably in the form of a compound such as hydrogenbromide or a hydrocarbon of bromine, which halogen reacts with tungstenvaporized from the filament 11 and returns it to the filament to preventblackening of the envelope 11, as is well known in the art.

In accordance with the present invention, the filament leg 13 and theportion of the spud wire 14 enclosed thereby are firmly bonded togetherby the mass of material indicated at 18 and consisting solely ofsintered tungsten particles. If desired, the tungsten bond may beextended over one or two turns of the filament leg 3 just beyond theinner end of spud 14 whereby to provide a more gradual temperaturegradient between the coil turns of the filament leg 3 adjacent to andoverlying the spud 14. This may be desirable in lamps containing bromineto minimize or prevent a tendency to an end attack or tungsten transferphenomenon which otherwise tends to occur between adjacent coil turnsacross which there is a steep temperature gradient, as more fullydescribed and claimed in application Ser. No. 609,592 of J. F. English,filed Jan. 16, 1967, now Patent No. 3,431,448.

The tungsten powder used to form the bond is extremely fine, preferablynot greater than about 5 microns size, and desirably about 0.8 micronaverage particle size, ranging from 0.77 to 1 micron, which is thefinest commercially available.

The tungsten resinate solution is preferably a 3.2% (by weight) tungstenresinate solution which is obtainable commercially from EnglehardIndustries, Inc., Hanovia Liquid Gold Division, Newark, N.J., anddesignated 3.2% Tungsten Resinate Solution #74. Such a resinate is madeby reacting the tungsten with a natural rosin obtained from trees. Theresinate is dissolved in a mixture of essential oil such as Oil ofRosemary and Oil of Lavender. The chief constituents of the rosin areresin acids of the abietic and primaric types having the general formulaC H COOH and having a phenanthrene nucleus. They are unsaturated andhence reactive.

The mixture or suspension preferably also contains a suitableheat-decomposable organic binder which is soluble in the tungstenresinate solution. The binder is preferably a cellulosic type, such asethyl cellulose. Where referred to hereinafter, it will be understoodthat the binder is ethyl cellulose of centipoise viscosity such as thatmarketed as Ethocell No. 100 viscosity, by Dow Chemical Company.

Good results have been obtained with a mixture or suspension prepared byadding 6 grams of ethyl cellulose binder (100 centipoise viscosity) to200 milliliters of 3.2% Tungsten Resinate Solution in a closed jar,stirring vigorously, for example by placing the jar on a magneticstirrer and allowing the binder to dissolve, (approximately threehours). 400 grams of 0.8 micron tungsten metal powder is added slowlywhile stirring by hand until the solution is thin enough to be handledelfectively by the magnetic stirrer, and is allowed to mix for one hourso that virtually each particle of tungsten powder is coated with thetungsten resinate solution. The ethyl cellulose binder is added toachieve a desired consistency, and the amount may be changed if adifferent consistency is desired.

The suspension is then applied to the filament joints where it soaksinto the coil leg 3 (or 13) by capillary action. In the case of the FIG.1 lamp, the filament assembly at that time includes the lead wires 6 andfilament 2 and its supporting structure prior to scaling it to the stem7. The assembly is placed in a furnace containing a non-oxidizingenvironment which might be a vacuum but is desirably a non-oxidizinggas, preferably hydrogen, where the filament connection is heated to atemperature in the range of about l600-l800 C., preferably 1700 C., fora time sufiicient to decompose and drive off the organic materials ofthe resinate solution and the binder, and leave a firm sintered metallicbond 8 consisting solely of tungsten.

The bond 8 of tungsten particles is different from ordinary sintering.The mixture or suspension shrinks about the connection, and thedecomposition of the tungsten resinate solution results in the formationof new tungsten which is probably highly reactive and effects a firmmolecular surface bonding of the tungsten powder particles to each otherand to the filament leg 3 (or 13) and the conductor 4 (or 14). Attemptsto sinter fine tungsten powder slurries without the resinate solutionwere to no avail; it did not adhere to the joint.

It will be evident to those skilled in the art that the proportions ofthe presently preferred composition described above may be variedsomewhat. For example, the 3.2% tungsten resinate solution may be variedbetween about 100 to 300 milliliters per 400 grams of tungsten powder.200 milliliters of the 3.2% tungsten resinate solution contains about5.3 grams of tungsten which is in a ratio, by weight, of about 1:75 tothe 400 grams of tungsten powder. For the stated range of resinatesolution, the weight ratio of tungsten in the tungsten resinate solutionto that of the tungsten powder is between 1:40 and 1:160.

Good results have been obtained by using a 22% tungsten resinatesolution in the above-stated proportions; however, the cost isconsiderably increased. Good results have also been obtained by asuspension containing 1 gram of ethyl cellulose dissolved in 25milliliters of 3.2% tungsten resinate solution to which was added 25grams of fine tungsten powder. The viscosity may be adjusted by additionof a suitable solvent, such as butyl acetate, or xylene, or toluene, ifnecessary.

The tungsten resinate solution cannot be used alone. It does not providesufiicient tungstein and does not stick; the tungsten powder is neededfor bulk. Sufiicient resinate is needed to bind the tungsten powderparticles together and promote sintering, presumably caused by theformation of new tungsten upon decomposition of the organic tungstenresinate.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. In an electric incandescent lam-p, the combination of a currentsupply conductor of refractory metal, a tungsten filament connected atone end to said conductor and bonded thereto by a mass of materialconsisting solely of tungsten which is the heat reaction product of asuspension of finely divided tungsten powder particles in a tungstenresinate solution and in which the particles are firmly sintered by asurface tungsten bond to each other and to said filament and saidconductor.

2. A lamp as set forth in claim 1 wherein the tungsten powder particlesare of a size not greater than about 5 microns.

3. A lamp as set forth in claim 1 wherein the tungsten powder particlesare of an average size of about 0.8 micron.

4. A lamp as set forth in claim 1 including a compact hermeticallysealed envelope containing the filament and also containing a smallquantity of halogen as a regenerative getter.

References Cited JOHN W. HUCKERT, Primary Examiner A. J. JAMES,Assistant Examiner U.S. Cl. X.R.

